1. Field of the Invention
The present invention relates to a method for fabricating a thin-film transistor, particularly to a method for forming a driver element of an active-matrix-drive liquid crystal display device.
2. Description of the Related Art
An active-matrix-drive liquid crystal display device and a direct-multiplex-drive liquid crystal display device are used as a display unit for a terminal of an information processing equipment or the like.
Because the active-matrix-drive liquid crystal display device independently drives each of many picture elements, it does not decrease the liquid-crystal driving duty ratio, contrast, or viewing angle differently from the simple matrix type even if the number of scanning lines increases due to increase of the information content to be displayed.
Therefore, the active matrix type provides color display equivalent to that of a cathode ray tube (CRT) and is more widely used as a thin flat display.
A thin-film transistor (hereafter referred to as TFT) is used as an element for driving a picture element electrode of the active-matrix-drive liquid crystal display device having the above advantage.
The TFT is divided into the stagger type shown in FIG. 1 and the inverted stagger type shown in FIG. 4 because of their structural difference.
The stagger-type TFT, as shown in FIG. 1, comprises a light-shielding film 2 formed in a TFT region on a transparent substrate 1, an insulating film 3 covering the light-shielding film 2, a source electrode 4s and a drain electrode 4d formed on the insulating film 3, a contact layers 5s and 5d formed on the source electrode 4s and drain electrode 4d, an active semiconductor layer 6 formed from the top of the two contact layers 5s and 5d to the region between them, and a gate electrode 8 formed on the active semiconductor layer 6 through a gate insulating film 7.
The light-shielding film 2 is formed to prevent light from entering a channel region layer of the active semiconductor layer 6 but it is unnecessary in some cases.
The insulating film 3 is formed between the light-shielding film 2 and the active semiconductor layer 6 so as to entirely cover the substrate 1.
The source electrode 4s and drain electrode 4d formed on the insulating film 3 are made of, for example, an indium-tin oxide film (hereafter referred to as ITO film). The contact layers 5s and 5d are made of an n+ amorphous silicon film (hereafter referred to as a-Si film) and phosphorus is introduced into the a-Si film. The active semiconductor layer 6 serving as the channel region is made of amorphous silicon.
The source electrode 4s and drain electrode 4d and the contact layers 5s and 5d are formed by pattering the ITO film and a-Si film 5. Patterning of them is, as shown in FIG. 2A, performed by using a patterned resist film 10 as a mask and etching the ITO film 4 and a-Si film 5 exposed from the resist film 10. In this patterning step, the ITO film 4 exposed from the resist film 10 is over-etched so that it does not remain on the insulating film 3. Therefore, as shown in FIG. 2A, at the ends of the source electrode 4s and drain electrode 4d, facing each other (portions A in FIG. 2A), the ITO film 4 constituting these electrodes is side-etched and therefore the a-Si film 5 on the ITO film 4 is overhung. The overhung film 5 causes the thickness of the a-Si film 6 serving as the active semiconductor layer 6 to decrease or the film 6 to be cut, causing the TFT to malfunction.
To prevent the above phenomenon from occurring, a method is considered to pattern the ITO film 4 and thereafter pattern the a-Si film serving as a contact layer as shown in FIG. 3A. For this method, however, it is difficult to adjust a pattern of the ITO film 4 to that of the a-Si film 5 and the ITO film 4 may be exposed at the channel region because these films are formed stepwise. And, as shown in FIG. 3B, when an a-Si film 7b serving as the active semiconductor layer 6 is entirely formed on the stepwise region, silicon on the ITO film 4 exposed from the contact layers 5s and 5d may be abnormally deposited and the abnormal deposition impairs transistor characteristics.
Even if the abnormal deposition is avoided, the resist or etching solution used to pattern the ITO film 4 may remain on the surface of the ITO film 4 or contact layers 5s and 5d. Thereby, the surface of the ITO film 4 or a-Si film 5 is contaminated. This causes a problem that the surface imperfectly contacts a film formed on the surface.
To solve the above problem, a method for selectively depositing silicon on a conductive film is proposed in the following literatures.
[1] G. N. Parsons, Appl. Phys. Lett. 59 (1991) pp. 2546-2548
[2] G. N. Parsons, IEEE Electron Device Lett. Vol. 13 (1992) pp. 80-82
In these literatures, an art is proposed to selectively deposit silicon on a source electrode and a drain electrode as a contact layer. If selective deposition is smoothly performed, neither overhanging nor stepwise region previously mentioned do not occur on an ITO film. The deposition temperature shown in these literatures ranges from 250 to 300xc2x0 C.
The inverted-stagger-type TFT, as shown in FIG. 4, comprises a gate electrode 12 formed on a transparent substrate 11, a gate insulating film 13 covering the gate electrode 12, an active semiconductor layer 14 covering the gate insulating layer 13, a channel protective coat 15 formed on the active semiconductor layer 14 above the gate electrode 12, and a source electrode 16s and a drain electrode 16d divided on the channel protective coat 15 and formed on the active semiconductor layer 14. Moreover, contact layers 17s and 17d are formed between the source electrode 16s and the active semiconductor layer 14 and between the drain electrode 16d and the active semiconductor layer 14. The gate electrode 12 and channel protective coat 15 are formed in almost same size.
The source electrode 16s and drain electrode 16d are made of a Ti film, the contact layers 17d and 17d are made of n+-type a-Si, the active semiconductor layer 14 is made of a-Si.
The following are steps of forming the source electrode 16s, drain electrode 16d, and contact layers 17s and 17d of the inverted-stagger-type TFT.
As shown in FIG. 5A, an n+-type a-Si film 17 and a Ti film 18 are laminated and thereafter a resist film 19 is applied onto the Ti film 18. Then, the resist film 19 is exposed by using an exposure mask PM and it is developed to form a pattern for forming a source and drain. Then, as shown in FIG. 5B, the Ti film 18 and n+-type a-Si film 17 are patterned by using a resist film 19 as a mask.
In this patterning, a margin is given to the pattern of the resist film 19 so that the pattern overlaps with the both sides of the channel protective coat 15 because it is difficult to adjust the edges of the source electrode 16s and drain electrode 16d to the edge of the gate electrode 12.
As a result, the gate electrode 12, as shown in FIG. 5B, has the width Lsd for securing a channel region and the margin width xe2x80x9cxcex94Lxc3x972xe2x80x9d where the source electrode 16s overlaps with the drain electrode 16d. Thus, the channel length Lg comes to a value close to Lg=Lsd+2xcex94L.
The parasitic capacity of the TFT increases due to the margin xcex94L. When the parasitic capacity increases, it is also necessary to increase a storage capacity connected to the TFT in order to prevent a static screen of a liquid crystal display device from being baked and the increase of the storage capacity causes the opening rate to decrease.
To solve the problem, a method is considered to form the source electrode 16s and drain electrode 16d in self-alignment by using the lift-off method.
For example, in FIG. 6A, an n+-type a-Si film 21 and a Ti film 22 are formed by leaving the resist pattern 20 used to pattern the channel protective coat 15 as it is, and thereafter the resist pattern 20 is removed as shown in FIG. 6B. As a result, the resist pattern 20, a-Si film 21, and Ti film 22 on the channel protective coat 15 are removed and a laminated film of the a-Si film 21 and Ti film 22 are left on the active semiconductor layer 18 at the both sides of the channel protective coat 15. Thus, the source electrode 16s and drain electrode 16d are formed at the both sides of the gate electrode 12 in self-alignment.
However, this method has problems that the residue of the resist pattern 20 attaches to the Ti film 22 at the both sides of the channel protective coat 15 and that of the a-Si film 21 and Ti film 22 removed from the channel protective coat 15 attaches onto the transparent substrate 11. Though the TFT is covered with a not-illustrated final protective coat, a plurality of picture-element electrodes may be shorted unless the residue of the resist pattern 20 is removed.
As a pretreatment for forming a film, a method for cleaning a base material by exposing it to hydrogen plasma is disclosed in the official gazettes of [3] U.S. Pat. No. 4,477,311, [4] U.S. Pat. No. 4,579,609, and [5] U.S. Pat. No. 4,849,375. However, because these literatures do not describe cleaning of a substrate and film when an ITO film is exposed, it is necessary to study the method before applying it to fabrication of a TFT.
It is an object of the present invention to provide a method for fabricating a stagger-type thin-film transistor to form an active semiconductor film which is not cut off in the vicinity of a channel region.
It is another object of the present invention to provide a method for fabricating a thin-film transistor to eliminate contamination in forming source and drain electrodes and clean the interface between layers from an active semiconductor layer to source and drain electrodes.
It is still another object of the present invention to provide a method for fabricating a thin-film transistor for efficiently and selectively depositing a contact layer formed between source or drain electrode and an active semiconductor layer.
The present invention selectively sticks a conductive material to the surface of source and drain electrodes of a stagger-type thin-film transistor and selectively deposits semiconductor by using the conductive material as growth species to form a contact layer.
Thus, no step shape occurs at the edges of the contact layer and the source and drain electrodes, and moreover, selectivity increases and the selective deposition efficiency is improved because the conductive material serves as growth species.
To form source and drain electrodes of an inverted-stagger-type thin-film transistor, a conductive material is selectively deposited on the surface of a contact layer to form source and drain electrodes with a film made of the conductive material. By selectively depositing source and drain electrodes, contamination of a thin-film transistor due to patterning is eliminated. The means for forming a contact layer includes a method for selectively forming an impurity-contained semiconductor film on an active semiconductor layer at the both sides of an insulting channel protective film and a method for introducing impurities into the active semiconductor layer.
For another invention of the present invention, when selectively depositing a semiconductor film serving as a contact layer by alternately repeating etching and deposition, the temperature for the etching is 200xc2x0 C. or lower and the deposition temperature is set to a value equal to that for the etching or higher. When lowering the temperature in the case of etching, the efficiency for selective deposition of a film is improved because the etching rate increases.
A contaminated layer on the surface of a semiconductor film serving as an active semiconductor layer and contact layer is removed by hydrogen or halogen plasma at the temperature of 200xc2x0 C. or lower. At this temperature, the film quality of the semiconductor layer is prevented from degrading and an electrode exposed from the semiconductor film is prevented from deteriorating due to plasma.
For still another invention of the present invention, when selectively forming an impurity-contained semiconductor film serving as a contact layer, the hydrogen or halogen content of a substrate insulator of source and drain electrodes is increased for a stagger-type thin-film transistor and the hydrogen or halogen content of an insulating film serving as a channel protective film is increased for an inverted-stagger-type thin-film transistor. Thus, in the case of selective deposition, the semiconductor film sticked on the surfaces of the substrate insulator and channel protective film is easily etched and the selective deposition time is decreased.